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[Xen-devel] Bug in Linux dom0 netback?

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Subject: [Xen-devel] Bug in Linux dom0 netback?
From: "Joakim Dahlstedt" <jda@xxxxxxx>
Date: Tue, 5 Feb 2008 09:52:18 -0000
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Thread-topic: Bug in Linux dom0 netback?
We are wondering whether someone can confirm that there is a bug in the
netback driver in dom0 Linux - that shows up in Xen 3.2.

We cannot create a domU that uses networking - as soon as we try dom0
reboots after a crash in net_rx_action.

In netbk_gop_frag we find the following code:
                if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                        new_mfn = alloc_mfn();
                         * Set the new P2M table entry before
                         * reassigning the old data page. Heed the
                         * comment in pgtable-2level.h:pte_page(). :-)
                        set_phys_to_machine(page_to_pfn(page), new_mfn);

                        mcl = npo->mcl + npo->mcl_prod++;

Notice !xen_feature(XENFEAT_auto_translated_physmap) and
Now in net_rx_action we find the following code

        if (npo.mcl_prod) {

As far as I can tell mcl_prod is only increased in netbk_gop_frag and
only if the auto_translated_physmap feature is not available. However,
in net_rx_action we will crash if the auto_translated_physmap feature is
not available - hence - if auto_translated_physmap is not available
netback always crashes.

We are not using HVM. 

In Xen 3.1 the code in net_rx_action looked like this:
        if (npo.mcl_prod &&

It seems like someone wanted to avoid the check in the if-statement and
turned it into a BUG_ON statement inside the if instead - but forgot to
invert the condition.

Can anyone confirm?

Best Regards,
  Joakim Dahlstedt

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